1. Field of the Invention
The invention relates to a measuring sensor comprising at least one SAW element serving as sensor and/or identification unit and having a housing part forming essentially a coaxial conductor into which the SAW element is installed in a contactless manner, and whereby said housing part is connected to the shield of the connection cable whose signal conductor extends in a contactless manner at least in the center across a superficial coupling structure of the SAW element, and whereby said signal conductor serves as near-field coupling of the electromagnetic field in the coaxial conductor with the SAW element.
2. The Prior Art
Measuring sensors of the aforementioned type are basically disclosed in AT 5.042 U2, for example, and have the advantage that the storage of partly very extensive sensor-relevant data can be stored away from the direct vicinity of the sensor with its high temperatures, vibrations and other interferences whereby only one identification unit with only a small number of identification data remains in or on the sensor, for example, with one identification sensor identification unit on the sensor itself having sensor recognition that can be correlated with the storage unit for sensor-relevant data. This small amount of identification data (e.g. a simple binary code) with its non-critical, simple and non-critical elements can be stored even in hostile ambient conditions and said data remains firmly connected with the sensor in a physical manner while any other sensor-related data (such as sensitivity curves, calibration data and the like) are present in the external storage unit whereby only examination of simple sensor recognition in terms of association is necessary to ensure the close relationship of sensor unit and sensor.
The design of the identification unit on the sensor can also have known SAW elements as mentioned in the beginning aside from the different simple design of the identification unit on the sensor as discussed also in the cited AT 5.042 U2. The use of surface acoustic wave (SAW) elements used as sensors and/or identification marks have been known for quite some time. Newer documents (Bruckner et al, Orc, 2003 IEEE Freq. Contr. Symp., 942 ff) have shown that such systems are especially suitable to be installed into existing technical systems as ID tags and/or for monitoring of physical or chemical parameters. It is thereby especially advantageous that such systems can be operated completely passive, which means no energy supply is needed as, for example, a battery in the region of the ID tag or the sensor. In addition, SAW elements are thermally and mechanically robust and they can be greatly reduced to miniature size and they can also be adapted in form and design for specific applications.
It is a state-of-the art and is disclosed in a great number of publications to couple SAW elements galvanic to the transmitting/receiving part or through electromagnetic far-field couplings by means of especially advantageous antennas for the respective application. It is also disclosed in EP 0827105 and EP 0502079, for example, to integrate a (transforming) coupling loop in the surface structures of the SAW element and to realize energy or signal transmission through inductive near-field coupling. The transmitting/receiving antenna is thereby a conductive loop designed and adjusted to coincide with the loop-shaped integrated antenna. These known solutions are sufficient and advantageous for a great number of technical applications. In contrast, not very well suited are the known coupling methods for installation of SAW elements into closed systems as, for example, pressure sensors for injection molding tools or for inner-pressure measurements of cylinders in internal combustion engines, especially if these are of compact design. The integration of SAW elements in such host-systems is of considerable more (potential) value, e.g. in the use of (passive) ID tags and/or sensors to monitor physical parameters (e.g. temperature, magnetic field forces, etc.) and/or chemical parameters (e.g. atmospheric components, thermal decomposition products, etc.)
All such uses have in common that the SAW system represents the completion of an existing system. This means that the entire SAW system is to be integrated into the host system whereby the entire SAW consists of a transmitting/receiving unit and the actual SAW element(s), in the rule without changes to the electric or installation/construction characteristics of the host system to be identified and/or to be monitored.
Coupling through a galvanic connection, which means a cable with a direct electric contact to the SAW element is not possible in many cases without influencing the actual useful signal of the host system in an inadmissible manner. Furthermore, a galvanic coupling in compact systems can be realized only with high costs in the production or assembly and/or it can be realized only limited in terms of being tolerant against high and/or changing temperatures, for example, especially over long periods, and against mechanical vibrations etc.
Coupling via an electromagnetic far-field coupling requires in the rule a massive constructive modification since a relatively large external antenna must be integrated as well as a free radio range between the transceiver and the SAW element, which often cannot be guaranteed in case of a sensor that is installed relative deeply in a metallic structure. Moreover, operating frequencies are limited to the (nationally different) ISM bands in such an arrangement, which therefore limits the maximal admissible transmitting power.
The third coupling method known from literature, which is an (inductive) near-field coupling between two congruent loops, is also not suited for practical application. The employment of an arrangement of this type would have, nevertheless, little or no negative influence on the useful signal and it requires no external antennas or long free radio ranges but it would need constructive changes through the greater space requirements for the internal coupling antenna as well as the required precise relative orientation of the SAW element and the coupling loop, if changes of this type would be possible at all or be admissible, and they would be complicated and expensive.
In total, a number of application suffer under these limitations of the practical application of SAW elements, e.g. for identification and monitoring functions. The invention intends to find a remedy hereby.
The basic design of many measuring sensors is similar to the aforementioned literature by Bruckner et. al (or as illustrated in FIG. 8 on page 470 of “Sensor 2003 Proceedings”)—(compare FIG. 1 of the aforementioned state-of-the-art). The active element 11 is connected to the control and reading element through the inner conductor 12 of a coaxial cable. The sensor housing 10 forms thereby a coaxial conductor 13, in a physical sense, with corresponding electromagnetic field distribution. One or several SAW element(s) 20 can be installed thereby in a contactless manner into the housing 10, which is preferably supplied through a high frequency coaxial cable, and the energy supply of SAW elements take place as well as the return-transmission of signals generated in the SAW elements, which is performed through near-field coupling of the electromagnetic field in the coaxial conductor with the SAW element.
An arrangement of this type has a number of advantages for the present application in comparison to other systems in the art. The coupling of the signal occurs via the coaxial cable of the host system whereby an otherwise necessary extensive modificantion of the host system can be avoided. The coupling occurs contactless so that falsification of the host-system can be effectively prevented. Since the field is generated in an electromagnetic closed space, it is possible and admissible to use a freely selectable frequency or a frequency range, which is especially advantageous for the SAW element and which does not impair at the same time the function of the host system. The transmission power can be adjusted to the system as desired to a great extent. Installation into the host system is finally made possible in a simple manner since no additional electrically conductive connections must be established in the host system and the requirements for precision in orientation are relatively low.
However, the disadvantage in the known measuring sensors of the aforementioned type is the fact that the coupling efficiency of the coupling structure integrated on the SAW element is relatively low so that the quality of the signal is often insufficient to ensure dependable identification or measurement data transmission under unfavorable ambient conditions and over lengthy periods.
A critical factor for the proper function of the measuring process is also the design of the coupling antenna on the SAW element. Systems known in the art with a coupling structure integrated on the SAW element are selectively designed having capacitive coupling, as illustrated in FIG. 3, or having a transforming (inductive) coupling. An encircling coupling loop, e.g. as presented in the aforementioned patent EP 0827105, is unsuitable for the present application since the induced voltage is cancelled out through the circular B-field in a coaxial conductor. In a solution in which a closed coupling loop 211 is arranged on one side of the inner conductor, as illustrated in FIG. 2, is functional in principal but provides insufficient coupling efficiency for practical application. The same limitation is valid for the capacitive coupling structure illustrated in FIG. 3.